Thermally Stable Perovskite Solar Cells by All-Vacuum Deposition

Vacuum deposition is a solvent-free method suitable for growing thin films of metal halide perovskite (MHP) semiconductors. However, most reports of high-efficiency solar cells based on such vacuum-deposited MHP films incorporate solution-processed hole transport layers (HTLs), thereby complicating prospects of industrial upscaling and potentially affecting the overall device stability. In this work, we investigate organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) as alternative, low-cost, and durable HTLs in all-vacuum-deposited solvent-free formamidinium-cesium lead triodide [CH(NH2)2]0.83Cs0.17PbI3 (FACsPbI3) perovskite solar cells. We elucidate that the CuPc HTL, when employed in an “inverted” p–i–n solar cell configuration, attains a solar-to-electrical power conversion efficiency of up to 13.9%. Importantly, unencapsulated devices as large as 1 cm2 exhibited excellent long-term stability, demonstrating no observable degradation in efficiency after more than 5000 h in storage and 3700 h under 85 °C thermal stressing in N2 atmosphere.

For fabricating solar-cell devices, the hole transport layers (HTLs) of Copper Phthalocyanine (CuPc) and Zinc Phthalocyanine (ZnPc), the perovskite layer, the electron transport layer C60, and the buffer layer Bathocuproine (BCP) were all evaporated in the same custom-built thermal evaporator. The chamber was pumped down to a base pressure between 8 x 10 -7 mbar and 2 x 10 -6 mbar for all depositions. The walls of the chamber were maintained at 17 C and the rotating substrate at 20 C through two separate chillers. Rates were monitored through gold-plated quartz crystal microbalances (QCMs) and a customised control software. During all depositions, QCM readings at each source and at the substrate were cross-checked. Each precursor material was individually calibrated on cleaned ITO substrates (or other underlying layers, where applicable) to determine the tooling factor, and hence, the actual deposition rate.
CuPc (Sigma-Aldrich, >99.95% trace metal basis, triple-sublime grade) was evaporated at a rate of 0.08 Å/s at temperatures between 320 C and 340 C until a layer thickness of 7.5 nm was achieved.
ZnPc (Lumtec, >99%, sublime grade) was evaporated at a rate of 0.08 Å/s at temperatures between 300 C and 330 C until a layer thickness of 7.5 nm was achieved. The Ag top contact with a thickness of 100 nm was evaporated in a separate Lesker Nano36 chamber. Using QCM readings, the initial rate was maintained at 0.2 Å/s for the first 10 nm, before ramping up to 1.5 Å/s.
The Au top contact with a thickness of 100 nm was evaporated in the same aforementioned Lesker Nano36 chamber. The initial rate was maintained at 0.1 Å/s for the first 10 nm, before ramping up to 0.7 Å/s.

Current-Voltage (J-V) Characterisation
Devices were measured under stimulated AM1.5G sunlight with an equivalent irradiance of 100 mW/cm 2 , generated by a Wavelabs Sinus-220 solar simulator and a Keithley 2400 source meter. The solar simulator was calibrated with respect to a KG-3 filtered silicon reference photodiode (Fraunhofer) prior to the measurement. Devices were characterised in ambient air condition at room temperature with relative humidity between 25% and 40%. The open-circuit voltage (Voc) was first measured for 3 s. Reverse and forward scans between -1.2 V and 0.2 V at a constant scan rate of 0.13 V/s were sequentially performed. Steady-state current and voltage were further probed for 30 s under continuous illumination, keeping the device close to its maximum power point (MPP) by actively tracking the maximum power point with a gradient descent algorithm. Finally, short-circuit current density (Jsc) was measured for 3 s. A mask was used for each substrate to separate the active area for each device to either 0.25 cm 2 or 1 cm 2 .
A spectral mismatch factor (M) was also estimated according to a previously reported method [1]. M was calculated to be 1.022 for devices with CuPc HTL and 1.020 for devices with PTAA and ZnPc HTL. We estimate the systematic error of this setup to be on the order of ±5% (relative). The mismatch factor has been applied to all power conversion efficiency (PCE) data points presented in this work.

Device Conditioning
As we note that the performance of devices tends to improve slightly with time, all device statistics shown in this work, except otherwise specified, were from a second J-V measurement taken between 7 and 11 days after the top metal contacts were deposited (i.e. completed fabrication). Meanwhile, PCE was calculated from the MPP of the reverse scan J-V curve for all data points, unless otherwise stated.

N2 Atmosphere Shelf-life Stability
Unencapsulated devices were stored in a glovebox under N2 atmosphere and under dim-light illumination. For this study, all first data points at 24 hours (h) or "day 1" equivalently denote PCE from MPP of the measured J-V curves tested 1 day after devices' completed fabrication. All further PCE data points were normalised with respect to their day 1 value.

Environmental and Operational Stability Testing
For all devices used in the following series of stability studies, devices were not placed in the specified testing conditions until 7 days after the top metal contacts were deposited, therefore allowing a second J-V measurement to be taken and be consistent with the rest of the device statistics. All further PCE data were normalised with respect to this measured PCE on day 7.

N2 Atmosphere 85 C Oven Stability
Unencapsulated devices were placed in a sample holder and covered by a metal lid inside a home-made oven maintained at (85  3)C. The oven was kept inside the glovebox under N2 atmosphere. Prior to every J-V measurement, all devices were retrieved from the oven and left inside the glovebox for additional 15 minutes to ensure they had returned to ambient temperature.

Ambient Air Stability
Unencapsulated devices were kept in dark in ambient air, at room temperature of (24  3) C, and a relative humidity between 30% and 40%.

Transmission -Reflection Measurement
Transmission -reflection measurements were performed on a Bruker Vertex 80v Fourier Transform Interferometer, with a tungsten-halogen near-infrared source, a CaF2 beam splitter, and a silicon diode detector. A blank z-cut quartz substrate and a silver mirror were used as the transmission and the reflection reference respectively. To calculate the absorption coefficient (α), the following equation was used: where T is transmission, R is reflection, and l is the thickness of the deposited film, which was deduced from stylus profiler measurements.

X-Ray diffraction (XRD) Measurement
XRD patterns were measured with a Panalytical X'pert powder diffractometer with copper x-ray source (Cu-K 1.54 Å set at 40 kV and 40 mA). All samples measured were deposited on ITO substrates, and all spectrum were further corrected with reference to the ITO peak at 2θ = 30.4.

External Quantum Efficiency (EQE) Measurement
EQE of fabricated devices was measured on a custom-built Fourier Transform photocurrent spectrometer with a Bruker Vertex 80v Fourier Transform Interferometer and a near-infrared source. Each device was held in place by a metal mask holder, so the exact same active area of 0.25 cm 2 (for the larger 1 cm 2 devices, only 0.25 cm 2 was sampled for the EQE measurement) as the J-V characterisation was illuminated. To calculate the EQE, the measured spectra was divided by the spectra of a calibrated silicon reference cell from Newport. For plotting the EQE spectrum, a smoothing function, taking the average of every nearest five data points, was also applied. To determine the Jsc from the spectrum EQE, the following integral was used, where q is elementary charge, is the wavelength, and 1.5 ( ) is the AM1.5 photon flux.

1.7
Steady-State Photoluminescence (PL) Measurement (1) (2) PL measurements were performed through photoexcitation of ITO/HTL/Perovskite thin films with a 398 nm continuous wave laser (PicoHarp, LDH-D-C-405M) with a power density of 6.38 W/cm 2 from the perovskite side. The emitted PL was coupled into a grating monochromator (Princeton Instruments, SP-2558) and measured with an ICCD camera (Princeton Instruments, PI-MAX4).

Time-Correlated Single Photon Counting (TCSPC) Measurement
TCSPC measurements were carried out through photoexcitation of ITO/HTL/Perovskite thin films with a 398 nm pulsed semiconductor diode laser (PicoHarp, LDH-D-C-405M) with a repetition rate of 10 MHz from the perovskite side. The emitted PL was coupled into a grating monochromator (Princeton Instruments, SP-2558) and collected by a photo-counting detector (PDM series from MPD). Timing was controlled by a PicoHarp300 event timer. Various excitation fluences were measured.
For fitting of the measured TCSPC transients, to account for the dual processes of charge transport into HTL and interfacial recombination, a double exponential in the ) was used.

Photoluminescence Quantum Yield (PLQY) Measurement
For PLQY measurements, ITO/HTL/Perovskite samples were placed in an integrating sphere and photo-excited by a 532 nm laser. The illumination intensity was 24 mW/cm 2 and equivalent to the half sun intensity. The signal was collected by a QEPro spectrometer.

Atomic Force Microscopy (AFM) Measurement
AFM measurements were carried out using an Asylum MFP3D (Asylum Research and Oxford Instruments Co.) in AC (tapping) mode. Olympus AC240-TS-R3 silicon tips were used for topography measurements.

Scanning Electron Microscopy (SEM) Measurement
FEI Quanta 600 FEG was used to take all SEM images. Prior to all measurements, the chamber was pumped down to high vacuum with a pressure less than 2x10 -4 mbar. For top-down SEM images, an acceleration voltage of 2 keV was chosen and current was defined by a spot size of 2.5. 2.2 Transmission -Reflection Data of PTAA Figure S2: Transmission (black) and reflection (red) spectra of the spin-coated PTAA thin film, deposited on ITO substrate and used as hole transport layer in this study.

Figure S3
: Unnormalised photoluminescence spectra of the ITO/HTL/FA0.83Cs0.17PbI3 halfstacks for three HTLs of PTAA, CuPc, or ZnPc. Laser excitation and photoluminescence collection were performed from both the "front" (i.e. direct excitation of the perovskite top layer) and the "glass side" (i.e. laser passing through the glass substrate, ITO and HTL first before exciting the perovskite). A 398 nm wavelength laser was used to excite the perovskite in all cases.

Figure S22
: Ambient air stability results for unencapsulated ITO/CuPc/FA0.83Cs0.17PbI3/ C60/BCP devices with Ag or Au top contacts. For devices with an active area of 0.25 cm 2 , the normalised average of 4 Ag devices and 6 Au devices (3 Au devices from 2180 hours) are illustrated. Prior to the ambient air stability testing, PCE of Ag devices were 13.7%, 13.8%, 13.5%, and 12.9%, while PCE of 0.25 cm 2 Au devices were 13.5%, 12.7%, 12.4%, 12.1%, 8.4%, and 6.8%. One normalised Au devices with 1 cm 2 active area is also plotted, which had an initial PCE of 12.3%.  Figure S23: XRD pattern illustrating full ITO/CuPc/FA0.83Cs0.17PbI3/C60/BCP devices with Ag or Au metal contacts aged in ambient air condition for 2180 hours, in conjunction with an ITO/CuPc/FA0.83Cs0.17PbI3 half-stack sample without any aging. For all three samples, the perovskite layer was deposited in the same batch. The X-ray source used was Cu-K 1.54 Å. For the Ag device with 2180 hours of ambient air aging, perovskite has predominantly degraded into the yellow delta-phase. For the Au device with 2180 hours of ambient air aging, both delta and tetragonal perovskite phases are observed.